What Size Air Conditioner Do I Need?

I'm a huge baby about temperature, and the fan just wasn't cutting it anymore. So in the midst of a recent heatwave I decided I was going to bite the bullet and get an air conditioner. But when I started researching online, I found myself confused by the various units' cooling and efficiency numbers, with prices seemingly directly correlated with each. But I didn't really know what any of them meant. Given the size of my apartment, how much air conditioner do I need?

"Many people buy an air conditioner that is too large, thinking it will provide better cooling. However, an oversized air conditioner is actually less effective — and wastes energy at the same time," explains the very helpful Energy Star website. "Air conditioners remove both heat and humidity from the air. If the unit is too large, it will cool the room quickly, but only remove some of the humidity. This leaves the room with a damp, clammy feeling. A properly sized unit will remove humidity effectively as it cools."

Luckily, they have a convenient chart for figuring how much AC power one needs for a room.

Area To Be Cooled (square feet)

Capacity Needed (BTUs per hour)

100 up to 150

5,000

150 up to 250

6,000

250 up to 300

7,000

300 up to 350

8,000

350 up to 400

9,000

400 up to 450

10,000

450 up to 550

12,000

550 up to 700

14,000

700 up to 1,000

18,000

1,000 up to 1,200

21,000

1,200 up to 1,400

23,000

1,400 up to 1,500

24,000

1,500 up to 2,000

30,000

2,000 up to 2,500

34,000

I was just going to put one in my living room, so I measured the room with a handy tape measure. It's not the most exact measurement in the world, but it's close enough. My living room is roughly 330 square feet, which the chart tells me means I need an AC unit with a capacity of 8,000 BTUs per hour, which was way less than I expected (here's a model which comes highly recommended).

Of course, a room's environment doesn't just depend upon square footage. In some cases there might be environmental issues to be aware of. For example, in very sunny rooms one increases the necessary capacity of an air conditioner by 10 percent. For very shady rooms, decrease it by 10 percent. If there are often more than two people in the room, add 600 BTUs per person, or make them bring ice.